A nuclear electromagnetic pulse ("Nuclear EMP") is a burst of electromagnetic radiation created by a nuclear explosion. The resulting rapidly varying electric and magnetic fields may couple with electrical and electronic systems to produce damaging current and voltage surges. The specific characteristics of a particular nuclear EMP event vary according to a number of factors, the most important of which is the altitude of the detonation.
(N)EMP Effects on Electronics
An energetic EMP can temporarily upset or permanently damage electronic equipment by generating high voltage and high current surges; semiconductor components are particularly at risk. The effects of damage can range from imperceptible to the eye, to devices literally blowing apart.
#Comment: Consider how much modern civilization relies on electronics and how only a hand full of NEMP's can take them all out.
Electromagnetic radiation hazards (RADHAZ or EMR hazards) (via)
Hazards caused by a transmitter/antenna installation that generates electromagnetic radiation in the vicinity of ordnance, personnel, or fueling operations in excess of established safe levels or increases the existing levels to a hazardous level; or a personnel, fueling, or ordnance installation located in an area that is illuminated by electromagnetic radiation at a level that is hazardous to the planned operations or occupancy.
If anyone poses a threat with electromagnetic pulse weapons, it's not Iran, but the US